179 Problems With Dating: H. naledi

Key Idea: The discovery of Homo naledi presents problems. The placement of the bones appears deliberate, which seems unlikely for an ancient hominin, and they are difficult to date because there are no dateable sediments around them. In 2013 two cavers exploring the Rising Star cave system near Johannesburg in South Africa found a passage that led to a chamber containing the bones of a new hominin species. Excavation has found more than 1500 specimens from at least 15 individuals including ribs, skulls, jaws, and dozens of teeth. Interestingly, no other types of animal or plant have been found in the cave and there is no indication of water flowing in the past. This has led the investigators to hypothesise that the bodies were deliberately placed there by other Homo naledi. Also the lack of other flora or fauna and sediments (other than cave dust) has made it very difficult to date the fossils. They have both advanced and primitive features, which some think puts them at the cusp of the transition between Australopithecus and Homo.

The Rising Star cave system

Cave entrance

Superman's crawl may have been higher in the past, allowing easier access. Bodies were probably dropped down the shaft over a long period of time (maybe centuries)

10 metres Superman’s crawl (less than ten inches high) Dragon’s back Dinaledi chamber

Fossil find

Dating Homo naledi

` Dating the Homo naledi bones is problematic because they were found deep in the cave. Ordinarily, fossils can be dated by relative dating. Other fossils in the sediment can be used to date the unknown fossils. For example, if the bones of a predator species of a known age had been found in the cave, then it may imply that H. naledi lived at the same time.

However, only H. naledi bones have been found, expect for a few small birds on the surface. Radiocarbon dating cannot be used because it only dates accurately to 50,000 years of age and H. naledi is likely to be much older than that. ` If the bones had been washed into the cave by a river system it may have been possible to use the sediments deposited or other bones that had been washed in to provide a date. The excavation team has yet to find any evidence of a river or water flow. One other way of dating the bones is by dating the flowstones found in the cave. Flowstones are sheets of calcium carbonate built up by water flowing down cave walls (similar to stalactites). However the flowstones do not cover much of the cave floor and fossils. ` Various anatomical ways of dating the fossils were tried, initially providing a date of ~1-3 my. However, more recent radiometric dating of specimens and the overlying flowstones has provided a much more recent date of 335,000- 236,000 years.

1. Describe one of the problems in explaining the deliberate placement Homo naldei bones in the Dinaledi chamber.

2. Describe two reasons why dating the H. naledi bones is difficult:

3D